Heavy duty machinery, trucks or trailers often have to receive grease in the gear box or transmission axle to do lubrication and prevent rusting. During maintenance, hardened grease has to be removed and fresh grease has to be replenished to make rotation of the gears or transmission axle easier, and also guard against rust. As the gear box usually has a complicated structure and a small space that make manual dispensing of grease difficult, a pneumatic grease gun has been developed to overcome this problem. Refer to FIGS. 1 and 2 for a conventional pneumatic grease gun. It has a gun body 1 to inject grease contained in a storage barrel 2 into the gear box or transmission axle to remove hardened grease and replenish fresh grease. Although it makes replenishing grease easier than the manual operation, it still has problems during operation, notably:
1. The conventional pneumatic grease gun has a piston 3 made of a pliable material. The piston 3 tends to wear off after moving to and fro for a period of time. As a result, air leakage occurs, and the thrust force to deliver grease decreases. When the thrust force is smaller than the returning elastic force of a reciprocal spring 4, grease cannot be dispensed effectively.
2. With the piston 3 of the conventional pneumatic grease gun wearing off, a portion of grease is squeezed back to the compressed air chamber 5 and clogs the air vane 6. As a result, the grease gun cannot function normally.